1. Field of the Invention
The invention concerns an installation for electrostatic application of a relatively good conductor coating product such as a water-based paint, for example; it is more particularly concerned with an arrangement for quickly changing coating product in the dead time between presentation of two objects to be coated.
2. Description of the Prior Art
In a coating product application installation in which the objects to be coated are carried by a conveyor past one or (usually) more sprayers there is usually a dead time between the end of application of a coating product to one object and the start of application of an often different coating product to the next object. This dead time corresponds to the distance between the objects on the conveyor. This is the case in the automobile industry in particular, where manufacturing constraints are such that two consecutive bodies on the conveyor generally have to be coated with different coating products. To be able to change the coating product for each object two parallel feed lines to the sprayer(s) are often used so that one can be cleaned and dried and then fed with the next coating product while the other is in use. This results in costly installations, especially for applying conductive coating products requiring autonomous storage tanks electrically insulated from ground.
The conductive coating products are supplied through long closed loop circuits between large storage tanks and the various spray booths. These closed loop circuits are grounded for safety reasons and electrical insulating means must be provided between the parts of the circuit which are grounded and those which are at the high-tension voltage during electrostatic application of the coating product.
The parts at the high-tension voltage include a small autonomous storage tank containing sufficient product to cover one object. Until now the best performing installations of this kind have been unable to execute all the cleaning and coating product change operations in a time less than the dead time defined above, which explains the need for two parallel feed lines.
Cleaning and in particular filling the autonomous storage tanks takes some time. Where several storage tanks must be filled simultaneously with the same coating product, for all the operations to be accomplished within said dead time the coating product circuit would have to supply the tanks in a few seconds with a sufficient quantity of product to feed the sprayers for an entire spraying period. The rate of filling might be approximately ten times greater than the average throughput of the feed to the sprayers. These flowrates would cause significant pressure losses in the feed circuits. What is more, the connections between the sprayer(s) and the coating product manifold connected to the various circuits by selector valves are sometimes relatively long (several meters) because the sprayers are distributed all around the path of the object to be coated. These connections are also of relatively small cross-section to render them flexible, said sprayers being mobile. Consequently, the connections would be affected by significant pressure losses. The autonomous storage tanks farthest from the feed circuits are therefore fed at only a fraction of the initial pressure, which increases the time to fill them.
For all these reasons it has generally been considered necessary to provide two parallel feed lines and to switch alternately between them.
The invention makes it possible to solve this problem by proposing a new arrangement enabling all the autonomous storage tanks to be filled very quickly.